Line an 8"-x-8" pan with parchment paper.

Set a large heat-proof bowl over a saucepan of simmering water to create a double-boiler. Add butter and stir until melted, then whisk in sugar, cocoa powder, egg, and vanilla until smooth. Continue whisking until mixture thickens slightly, 2 to 3 minutes more.

Remove bowl from heat and fold in graham cracker crumbs, coconut, pecans, and salt. Transfer mixture to prepared pan and press into an even layer. Transfer to refrigerator and chill for 15 to 20 minutes, or until firm.

Meanwhile, make the buttercream layer: in a medium bowl, using a hand mixer, cream butter and custard powder or pudding mix together until smooth. Add powdered sugar and salt and beat until well-mixed and mixture is like coarse sand, then add heavy cream one tablespoon at a time, beating well after each addition (Or add hot water all at once). Spread buttercream mixture over crust and refrigerate until firm, 25 to 30 minutes.

Make chocolate layer: combine chocolate chips and butter in a microwave safe bowl. Microwave on medium power for 30 second increments, stirring between each round, until chocolate and butter are completely combined and smooth. Pour mixture over chilled bars smooth into an even layer. Sprinkle with flaky sea salt and refrigerate until chocolate layer is smooth, 15 to 20 minutes (recommend slicing top chocolate layer after about 10min (or let sit out 10min after it cools in fridge) since the chocolate may crack if you wait too long). Cut into squares when ready to serve.